The goal of the proposed research is the further elucidation of the functional organization of the mammalian spinal cord, utilizing electrophysiological and morphological techniques. The main thrust of the research effort at present is in the identification of specific categories of spinal cord interneurons and in the study of their response characteristics. The experiments planned can be grouped under the following headings: (1) determination of the localization of the cell bodies and axons of the neurons giving rise to the spinothalamic tract and the description of the responses of these cells to electrical and natural stimuli; (2) similar work on other ascending tracts, including the spinocervical tract and the spinoreticular tract; (3) an investigation of the effects of group I activated interneurons of the intermediate nucleus upon spinal motoneurons and a further study of their responses to muscle receptor activation in the unanesthetized cord.